03-Loops.md

(FOR, FOR /L, FOR /F)

• 📌 Overview of FOR Variants

  Syntax	Purpose
  FOR %%A IN (...) DO	Loop over a set of items/files
  FOR /L %%A IN (start,step,end) DO	Loop through numeric ranges
  FOR /F %%A IN (...) DO	Loop through text files or command output

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1. FOR %%A IN (...) DO

• ✅ Syntax

  FOR %%A IN (item1 item2 item3) DO (
      ECHO Item: %%A
  )

• 📁 Use Case: Iterate Files

  FOR %%F IN (*.txt) DO (
      ECHO Found text file: %%F
  )

  • Wildcards like * and ? are supported.
  • Double percent (%%) is used in batch files. In the command line, use single %.

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2. FOR /L %%A IN (start,step,end) DO

• This variant is similar to traditional C-style loops.

  ✅ Syntax

  FOR /L %%I IN (1,1,5) DO (
      ECHO Count: %%I
  )

  • (1,1,5) means start at 1, increment by 1, until 5.
  • You can count down too:

  FOR /L %%I IN (5,-1,1) DO (
      ECHO Reverse Count: %%I
  )

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📄 3. FOR /F %%A IN (...) DO

• Processes contents from:

  • A file
  • A string
  • The output of a command

  ✅ Reading Lines from a File

  FOR /F "tokens=1,2 delims=," %%A IN (data.csv) DO (
      ECHO Name: %%A, Age: %%B
  )

  Options:

  • tokens=n: Select which fields (words) to parse.
  • delims=,: Define delimiter character.
  • skip=n: Skip header lines.

  📜 Parsing Command Output

  FOR /F %%A IN ('DIR /B *.txt') DO (
      ECHO File: %%A
  )

  ---

  🔁 Nested Loops

  FOR %%I IN (A B) DO (
      FOR %%J IN (1 2) DO (
          ECHO %%I %%J
      )
  )

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⚠️ Gotchas & Tips

• Always double the % sign in batch files (%%A).

• Wrap variables in quotes to handle spaces.

• When reading from command output, FOR /F trims whitespace by default.

  To preserve spacing and empty lines, use usebackq and delims=.

  FOR /F "usebackq delims=" %%L IN ("my file.txt") DO (
      ECHO Line: %%L
  )

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🧪 Real-World Examples

1. Rename All .log to .bak

   FOR %%F IN (*.log) DO (
       REN "%%F" "%%~nF.bak"
   )

2. Read IPs from File and Ping

   FOR /F %%I IN (ips.txt) DO (
       PING -n 1 %%I
   )

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🧠 Summary Table

Type	Example	Use Case
File loop	FOR %%F IN (*.txt)	Process file list
Number loop	FOR /L %%I IN (1,1,5)	Count or iterate numerically
Text loop	FOR /F %%L IN (file.txt)	Read lines or command output